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I've been looking into heating a nickel chromium wire with a 9V battery for one of my projects. Can anyone feed me an equation for calculating the life of the battery? I'm sure it will include gage of wire, length of wire, resistance, etc. I can find out these aspects if I can get the general equation. Thanks in advance.

W. Vacek
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    Also depends on battery chemistry/capacity, internal impedance, ambient temperature & airflow across the wire. Will be very difficult to come up with an accurate simple equation, but you might get somewhere in the ballpark. – John D May 23 '16 at 18:00
  • your question needs to be much more specific, if you can show you have already thought of the detail, you are more likely to get a useful answer. – Sean Houlihane May 23 '16 at 18:13
  • General equation = Capacity in mAh / Current Draw in mA – Passerby May 23 '16 at 18:17

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You must know the current drawn by your load (your heater coil). Ohm's Law calculates the current (Amps) = Voltage divided by resistance (ohms).

Then you must know the capacity of your battery. This is typically stated in Amp-Hours (Ah) or mili-Amp Hours (mAh) for small batteries. Those little rectangular 9V batteries have capacities ranging from 175 mAH (NiMH) to 1200 mAh (lithium).

So then it is a rather simple matter of solving the equation for the current drawn, and the capacity of the battery.

Then you must also know what is your "cutoff" point at which the battery can no longer generate enough heat for your system.

Note that those little 9V rectangular batteries have the LOWEST capacity of all the commonly available consumer batteries. Even less than a AAA cell.

Richard Crowley
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